1. Field of the Invention
The present invention relates to a method and an apparatus for the joint storage and/or transfer and/or transportation of low-temperature liquefied combustible gas, in particular natural gas, and of liquid nitrogen and also to uses of the method and of the apparatus.
2. Related Art
It is known to operate internal combustion engines of motor vehicles with natural gas. The natural gas is carried, either under pressure or in low-temperature liquefied form, in special storage tanks in the motor vehicle. In the latter case, the liquid natural gas has a temperature of at most −161° C. at atmospheric pressure, and part of the natural gas may constantly evaporate on account of the heat supplied by the surroundings. Evaporation is undesirable, especially when the motor vehicle is not in operation. The gaseous part of the natural gas mainly contains the more volatile constituents of the gas which have a higher vapour pressure or a lower boiling temperature. The extraction of part of the natural gas present in gaseous form in the storage tank leads, in the long term, to an enrichment of higher hydrocarbons, in particular propane, in the low-temperature liquefied part of the natural gas. The high propane gas fraction may be harmful for an internal combustion engine, when later supplied to it. Moreover, the mixture composition changes with time, this being undesirable. In an extreme case, the higher hydrocarbons may also freeze out of the solution.
In refrigeration vehicles operated with natural gas, which have a refrigeration space for the transport of refrigerated stock, it is known from EP 0 788 908 A2 to utilize the evaporation energy of the natural gas to keep the refrigeration space at a low temperature. The gaseous natural gas is subsequently supplied to an internal combustion engine designed for natural gas. At times when the vehicle is not being operated, but the refrigeration space has to be kept cold, the refrigeration space is cooled conventionally, in which case the conventional cooling engine may, in turn, be driven by natural gas. Among other things it is disadvantageous that the natural gas itself needs to be conducted all the way from a natural gas tank mostly close to the combustion engine to the cooling unit, usually arranged in or close to the refrigeration space, which implies a lot of risks according to the flammable properties of the natural gas in case of defects like leakages in the supplying fuel lines. Moreover, especially the use of such a cooling method in a trailer truck is only applicable quite limited, because no appropriate, workable systems for disconnecting fuel lines, which would be necessary in case of decoupling a trailer from a tractor, are available. Therefore the refrigeration space of a trailer in a trailer truck usually is cooled by liquid nitrogen or by conventional compressor cooling methods solely, even if the tractor is driven by natural gas.
Evaporating natural gas also represents a problem at filling stations which make liquid natural gas available for natural-gas-operated vehicles. According to the prior art, the evaporated gaseous natural gas is compressed in a complicated way at the filling stations and may be used further, where appropriate, in gaseous form. However, here too, the result of the constantly evaporating natural gas is that the liquid natural gas still contained in the tank is enriched after some time with higher hydrocarbons, in particular propane, since the constituents having a lower boiling point evaporate sooner. The rising propane fraction may, beyond certain limits, constitute a problem for natural-gas-operated internal combustion engines.